Overexpression of BCL-X-L underlies the molecular basis for resistance to staurosporine-induced apoptosis in PC-3 cells

We have reported previously that among human prostate cancer cell lines LNC aP but not PC-3 cells undergo apoptosis after treatment with the protein ki nase inhibitor staurosporine (STS). We have now further investigated this m odel to uncover the molecular mechanism causing resistance to STS-induced a poptosis in PC-3 cells. S-100 lysates of both cell lines showed biochemical changes typical of apoptosis after the addition of cytochrome c and dATP, suggesting that the postmitochondrial phase of apoptosis was intact. Upon a ddition of STS, the proapoptotic molecules Bax and Bad became predominantly mitochondrial in both cell lines. This, in turn, was followed by loss of m itochondrial transmembrane potential, translocation of cytochrome c to the cytosol, activation of caspase-9, -3, and -7, and cleavage of the apoptotic targets, DNA fragmentation factor and poly(ADP-ribose) polymerase, in LNCa P but not in PC-3 cells. Components of the mitochondrial. permeability tran sition pore, adenine nucleotide transporter and voltage-dependent anion cha nnel, were normally expressed in the correct subcellular fraction of both c ell lines. Overexpression of the proapoptotic proteins Bax and Bad, fused t o a green fluorescent protein but not of green fluorescent protein alone, i nduced apoptosis in >80% of PC-3 cells. These experiments suggested that a factor protecting the mitochondria of PC-3 cells mediates resistance to STS -induced apoptosis. A wide search among the antiapoptotic Bcl-2 family memb ers was performed, and Bcl-X-L was found to be overexpresseAd in PC-3 cells . Experiments down-regulating Bcl-X-L expression by using the tyrosine kina se inhibitor genistein, sodium butyrate, or an antisense Bcl-X-L oligonucle otide restored sensitivity to apoptosis in PC-3 cells, Thus, Bcl-X-L overex pression is one of the mediators of resistance to STS-induced apoptosis in the prostate cancer cell line PC-3.